Relationship between Shaft Voltage and Bearing Grease in Brushless DC Motor Driven by PWM Inverter

Abstract

This paper describes the relationship between the shaft voltage and the bearing grease in a brushless DC motor driven by a PWM inverter. The motor has an insulated rotor to reduce the shaft voltage. At first, we establish a method that determines the bearing capacitance by using the proposed bridge-type equivalent circuit and calculates the shaft voltage from the equivalent circuit. Next, we establish a method calculating the dielectric breakdown voltage of a bearing from the oil film thickness of the bearing grease. Finally, we compare the calculated dielectric breakdown voltage with the shaft voltage of the motor. In addition, we confirm the validity of the insulated rotor in the motor by clarifying the fact that in the motor the shaft voltage is less than or equal to the dielectric breakdown voltage of the bearing grease over the practical operating speed range.